1. Field of the Invention
The present invention relates to an integrated ion sensor, and more specifically, to an integrated ion sensor for measuring an electrolytic component and the like in a solution such as blood, urine, water in a river, and the like.
2. Description of the Related Art
An ion sensitive FET (ISFET) as an ion sensor making use of a MOSFET is generally arranged such that an ion selective membrane is directly formed on a gate insulation film without forming a gate electrode. A measuring circuit making use of an ISFET having this arrangement is, for example, described in IEEE Transactions on Electron Devices, vol. ED-26, No. 12 (1979), pp 1939-1944. The measuring circuit described in this literature needs a power supply for making a voltage and current between the drain and source of the ISFET constant, respectively and a power supply for imposing a proper voltage on a reference electrode provided for the purpose of making a sensor circuit active to process a signal.
Since the measuring circuit using the ISFET arranged as described above needs the two power supplies for the sensor circuit and the reference electrode, the number of wirings is increased and thus a device is made large and complex. Further, when the ion sensor is composed of a plurality of sensors each having a different ion selective membrane and simultaneously measures a plurality of different kinds of ions in a solution by using the sensors, although one reference electrode is commonly used for the sensors, the ion sensor needs measuring sensor circuits that are as many as the sensors, which makes the arrangement of the ion sensor more complex. In particular, when the above ion sensor is arranged such that the sensor circuits and reference electrode are assembled in a catheter and the ion sensor is miniaturized to measure in vivo in an organism, a disadvantage arises in that the number of lead wires to be connected to an external power supply and measuring circuit is increased.
While a preferred embodiment has been set forth with specific details, further embodiments, modifications and variations are contemplated according to the broader aspects of the present invention, all as determined by the spirit and scope of the following claims. Sensors and Actuators, 4, (1983), pp 291-298 discloses the arrangement of another ISFET, which transmits a signal regarding an ion concentration detected by the ion selective membrane to the gate electrode of a MOSFET through a signal line, as shown in FIG. 2 of the literature.
When the ion sensor having the ion selective membrane electrically connected to the gate electrode of the MOSFET through the signal line composed of, for example, polysilicon, as described above, is used for a long time, water molecules and the like in a solution to be measured pass through the ion selective membrane and enters the signal line and gate electrode to oxide or dissolve them. Accordingly, the state of the interface between the ion selective membrane and the signal line or gate electrode is made unstable, and thus a disadvantage arises in that the change in the responsive characteristics of the ion sensor caused by an elapse of time is increased.
Conventionally known is a voltage follower circuit serving as an electric circuit arrangement having a gain set to 1. When the voltage follower circuit has two MOSFETs provided at the input stage thereof, an ion sensor including an ISFET and generating an output with a gain of 1 can be made by replacing one of the MOSFETs with the ISFET. To actually realize a voltage follower circuit generating the above output, the structural characteristics, i.e., the thickness of the oxide film of a gate, channel length, channel width and the like of a MOSFET to be disposed on an input stage must strictly coincide with those of an ISFET to be disposed thereon. Since, however, the ISFET having the ion selective membrane electrically connected to the gate electrode through the signal line has the great change in the responsive characteristics caused by an elapse of time, as described above, when the ion sensor is composed of the voltage follower circuit, the characteristics of the ISFET does not coincide with those of the MOSFET when the ion sensor is used for a long time. As a result, since the gain of the circuit is offset from 1 and thus the output from the ion sensor must be corrected, the circuit arrangement is made complex due to the provision of a corrective circuit.